Some BRI devices in France insist on checking TEI's when in point-to-point
mode. If they don't get a response for TEI 0 they drop layer 1 even
though libpri keeps trying to bring layer 2 up.

* Made q921_mdl_receive() handle TEI check request messages in TE-PTP
mode. Had to change q921_mdl_send()/Q921_INIT() because the PTP modes do
not setup a link structure specifically for MDL as the PTMP modes do.

* Fixed q921_tei_check()/t201_expire() to check TEI's even if the network
side doesn't have any assigned. This should make TE's that request the
TEI verify procedure (Q.921 Section 5.3.5) happy when the network side
doesn't have any TEI's allocated.